1. Field of the Invention
The present invention relates generally to a fluid-filled elastic mount having a fluid chamber partially defined by an oscillating plate which is actuated by an electromagnetic drive device so as to change the fluid pressure in the fluid chamber to thereby control the damping characteristics of the mount. More particularly, the present invention is concerned with such a fluid-filled elastic mount which has an electromagnetic drive device capable of stably producing a large drive force to actuate the oscillating plate.
2. Discussion of the Prior Art
As a vibration damper for flexibly connecting two members in a vibration system or mounting one of the two members on the other member in a vibration damping fashion, there is known an elastic mount interposed between the two members of the vibration system. The elastic mount has an elastic body interposed between and elastically connecting a first and a second support which are respectively fixed to one and the other of the two members of the vibration system. This type of elastic mount may be used as an engine mount or a suspension bushing for a motor vehicle, for example.
Recently, there have been proposed various types of fluid-filled elastic mounts adapted to exhibit sophisticated damping characteristics, wherein the elastic body which elastically connects the first and second supports partially defines a fluid chamber filled with a non-compressible fluid. Some of these fluid-filled elastic mounts are adapted to electrically control the fluid pressure within the fluid chamber, depending upon the type of the input vibrations received, so that the specific vibrations can be suitably damped or isolated.
Examples of such electrically controllable fluid-filled elastic mount are disclosed in JP-A-59-1828, JP-A-59-1829 and JP-U-3-73741, wherein the fluid chamber is partially defined by an oscillating plate, which is oscillated by an electromagnetic force produced by a permanent magnet and a coil, so as to-suitably control the fluid pressure within the fluid chamber, to thereby enable the elastic mount to exhibit different vibration damping characteristics depending upon the type of the input vibrations received.
In the known electrically controllable fluid-filled elastic mount constructed as described above, the operation of the oscillating plate cannot be suitably regulated so as to enable the elastic mount to exhibit satisfactory damping characteristics, since it is difficult to obtain a sufficiently large drive force to effectively actuate the oscillating plate.
Described more specifically, the above fluid-filled elastic mount suffers from insufficiency of the magnetic flux density in the magnetic field in which the coil is placed, because the magnetic circuit or path formed by the permanent magnet is open. In particular, the open magnetic circuit leads to insufficiency of the drive force to actuate the oscillating plate so as to effectively regulate the fluid pressure within the fluid chamber, when the elastic mount receives a vibrational load of medium to low frequencies having a relatively large amplitude.
Moreover, the open magnetic circuit formed by the permanent magnet is unable to provide a substantially constant magnetic flux density in the field in which the coil is placed, and therefore inevitably causes a large variation in the magnetic flux density applied to the coil when the oscillating plate is actuated or displaced in the oscillating manner. As a result, the drive force which acts on the oscillating plate tends to be unstable, making it difficult to effectively control the oscillating plate, whereby the waveform of the pulsation induced within the fluid chamber is distorted, causing a fluid pressure control distortion of the fluid chamber. Thus, the known electrically controllable fluid-filled elastic mount is not satisfactory in terms of its damping characteristics.